1. Field of the Invention
This invention relates to loop driven vehicles including a frame, a power unit mounted on the frame, a wheel driven by a driving loop, e.g. a chain entrained about driving and driven sprockets on the wheel and on the power unit respectively, the wheel being mounted in a pivoted fork whose movement is controlled by resilient suspension means operative between the frame and the fork.
The invention is particularly applicable to motor cycles.
2. Description of Prior Art
It has been proposed hitherto to mount the power unit of a motor cycle relatively to the frame by resilient mountings in order to isolate, as far as possible, the rider from vibration arising from operation of the power unit. Vibration transmitted to the frame and thus to the rider of a motor cycle can become extremely disturbing to the rider over a period of time. Despite an increasing tendency nowadays to install multi-cylinder power units in motor cycles, which power units can be designed to give little vibration when operating, single and twin cylinder engines are still widely employed, particularly in smaller machines, and as these engines inherently tend to give rise to relatively large levels of vibration when operating there is a requirement for mounting such power units in a manner providing for damping of vibrations transmitted from the power unit to the frame.
In U.S. Pat. Nos. 3,542,146 and 3,783,961 there are described motor cycles in which the pivoted fork which supports the rear wheel is pivotally mounted on the power unit, and the power unit is resiliently mounted to the frame by mountings each of which permits much greater relative movement between the power unit and frame in directions parallel to a plane perpendicular to the axis of rotation of the driving wheel than in directions parallel to said axis. By this means, accurate alignment between the driving and driven sprocket on the power unit and wheel is maintained, to ensure satisfactory operating conditions for the driving chain. If such alignment is not maintained, the chain may become disengaged from one or both sprockets. Vibration damping is obtained by virtue of the permitted movement of the power unit parallel to the plane perpendicular to the driving wheel axis, and the restriction of movement of the power unit in directions parallel to said axis ensures that the driving wheel is maintained in the correct alignment with the frame. Retention of this alignment is essential in order that a motor cycle retains satisfactory handling qualities.
It has been found in practice, however, that the systems described in these two U.S. patent specifications require considerable maintenance attention if the machine is to maintain satisfactory handling qualities. This is because the mountings incorporate cooperating faces which determine permissible movement of the power unit in directions parallel to the driving wheel axis, which faces are subject to wear with the result that the required alignment between the driving wheel and frame may not be satisfactorily maintained under all conditions if maintenance has been neglected.
A further system has been proposed in U.S. Pat. No. 3,722,612, which system eliminates the problem of required maintenance to maintain the handling qualities of the motor cycle. In order to ensure vibration damping, however, this system requires that the engine should have unbalanced reciprocating loads, which greatly increases the loading on the main bearings of the engine. Although this relatively increased main bearing loading can be acceptable in certain two stroke cycle engines, it is in general unacceptable for four stroke cycle engines.